This invention relates to electrocoating a container. More particularly, the invention relates to an electrode probe and method for use of the same for controlling the flow of electrocoating material into the container to be coated.
Usually, metal containers, such as cans and the like, have their interior surfaces and exterior surfaces coated with protective materials, such as resinous coating materials. The interior coating is required for protection of the container from its contents and protection of the contents of the container from reaction with the metal. An exterior coating may improve handling of the container and/or its aesthetic appearance. Additionally, an exterior coating can offer protection against the container environment, such as by inhibiting the rusting of steel and tinfree steel containers in moist atmospheres and forming of excessive oxide on aluminum containers during retorting.
Though the coating materials may be applied by sprays, rolls, immersion or the like, electrocoating techniques may be the most desirable since they can provide uniform and consistent films. A process and apparatus for uniformly electrocoating a container is disclosed in each of U.S. Pat. Nos. 3,922,213 issued Nov. 25, 1975 and 4,094,760 issued June 13, 1978, to the common assignee of the present invention. The method and apparatus of both patents, the disclosures of which are incorporated herein by reference, considerably improve the techniques for electrocoating metal containers in high speed production lines by avoiding the slower prior art processes. U.S. Pat. No. 3,922,213 relates to the uniform electrocoating of the interior of a shaped metal container, while U.S. Pat. No. 4,094,760 is an improvement thereover which permits the electrocoating of both the interior and exterior of a metal container simultaneously. Both patents disclose uniformly electrocoating the metal container in an inverted position by the insertion of an electrically conducting probe-nozzle therein through which coating material is flowed into the interior of the container to fill the container and maintain a transient bath of coating material therein. An electrical potential is impressed between the container and the nozzle to coat the metal surfaces.
The profile of a container bottom end wall can be a complex geometry with bends and curves having various radii of curvature. For example, efforts by the can making industry to produce light weight containers, such as drawn and ironed cans, have resulted in thin gauge container bottoms having a plurality of concave and convex surfaces forming recesses and "corners" with small radii contributing to container wall strength. In addition to designing end wall profiles for container strength, such profiles are also designed for aesthetic appearance and have made electrocoating the small radii "corners" of the metal containers increasingly difficult. Even the process and apparatus of the two above-cited patents which fill a container with a transient bath of electrocoating material have occasionally resulted in coating voids in areas of the small radii of the container bottoms. Coating voids result when the "corners" are not wetted with coating material. Thus, there is a need to improve an electrocoating electrode probe to flow electrocoating material into the small radii of the container bottoms and to improve the "throwing power" of the electrode probe to uniformly electrocoat container interiors.
It is known in the art of coating to insert nozzles into container bodies for coating the same wherein the nozzles have a plurality of openings such that coating material is directed in a specific pattern. U.S. Pat. No. 3,643,727, issued Feb. 22, 1972, discloses a process and apparatus for electrostatic spray coating the interior of a conductive metal tube as the tube is being extruded. A probe includes a nozzle having coaxial spray ducts of annular section angled outwardly with respect to the axis of the probe and additional openings on the periphery of the probe connected with exhaust ducts for exhausting excess coating material from the interior of the metal tube. U.S. Pat. No. 3,410,250, issued Nov. 12, 1968, discloses a spray nozzle for applying a uniform coating with two streams of thermosetting resin within a container interior. A venturi-type opening having a beveled outer annular section provides a low density spray to the closed end of the container and orifices on the periphery of the tubular member of the nozzle direct high density spray substantially radially outwardly from the tubular member. A nozzle for spraying fine powder suspended in a divided flowing gas stream is disclosed in U.S. Pat. No. 3,422,795 issued Jan. 21, 1969. The nozzle orifice has an expanding horn with parabolic surfaces which diverts coating material primarily to the container bottom and corners. The outside surfaces of the expanding horn are to direct coating material outwardly with minimum disturbing effect on the laminar flow. It is also known to use spray nozzles for producing fan-shaped patterns to match internal surface configurations, such as is shown in U.S. Pat. No. 3,737,108, issued June 5, 1973, and U.S. Pat. No. 2,964,248, issued Dec. 13, 1960.
Further, it is known that a flow-through electrode may have a plurality of orifices evenly spaced about its periphery, as shown in U.S. Pat. No. 3,399,126, issued Aug. 27, 1968, which discloses an apparatus for electro-deposition using a plurality of conduit electrodes submerged in a bath of coating material for directing streams of coating material against selected surfaces of a submerged article having a complex configuration.
Such prior art nozzles for both atomizing and airless spray coating are inadequate for several reasons for processes using a flowing bath of electrocoating material. Maintaining a "fill" of flowing electrocoating material in a container is essential, as well as avoiding any discontinuities of flow, such as created by air pockets and excessive turbulence. Such prior art nozzles, which develop specific spray patterns, not only promote discontinuities, but also are inconsistent with a process of a transient flowing bath of electrocoating material where a fill is maintained to completely wet the container surfaces to be coated. Further, nozzles useful in slower prior art processes for electrocoating articles submerged in a bath of coating material are not suitable for high speed production, such as is typical in the can-making industry. There exists a need, therefore, for a probe-electrode of improved "throw power" for controlling the flow of electrocoating material into the deep recesses and corners of containers without creating discontinuities of flow so as to uniformly electrocoat the container.